It has long been a goal of engineers and scientists to develop more effective methods and apparatuses for harvesting the kinetic energy that is present in naturally occurring flows of wind, water or flows of other material. To accomplish this, windmills and turbines have been developed with blades that are joined around a central hub and shaped so that kinetic energy from a fluid flow against the blades urges the blades to rotate about the hub. The velocity and torque with which the hub rotates provides rotational energy that can then be used for other purposes. In some cases, the rotational energy provided at the hub is supplied directly to mechanical systems that perform work such as milling or pumping water. However, most modern windmills and turbines connect electrical generators to the hub to convert rotational mechanical energy into electrical energy.
The blades used in windmills and turbines spin in a direction that is generally normal to a direction of the fluid flow and can have disruptive effects on the fluid flow and on things carried by the fluid flow. The spinning blades are also subject to static and dynamic balance problems. For example, when dynamic unbalance conditions exist, rotation of the blades urges the hub to rotate about an axis other than predetermined rotational axis of the hub. Mountings such as bearings that are used to position the hub for rotation about the predetermined rotational axis provide reaction forces that resist the urging. This reduces the efficiency of the energy conversion process and causes premature bearing failure which increases maintenance costs. These effects in turn increase the cost of electricity produced by windmills and turbines. Additionally, such reaction forces can create noise and vibration in the windmill or turbine that can have a disruptive effect in the area surrounding the windmill or turbine.
What are needed therefore, are methods and apparatuses that allow for the generation of electrical energy in a manner that is less disruptive to the flow of fluid and to the surrounding environment. What is also needed, are methods and apparatuses that generate energy with lower maintenance requirements.
In some cases, efforts have been made to meet these needs through the use of piezoelectric webs. For example, U.S. Pat. No. 6,424,079 issued to Carroll on Jul. 23, 2002 and entitled: “Energy Harvesting Eel” describes a piezoelectric power generator for use in a fluid flowing stream. This piezoelectric power generator has an elongated flexible central layer of a dielectric material with axially along opposite sides thereof a plurality of separate piezoelectric elements each formed from a portion of a continuous layer of the piezoelectric layer extending along each opposite side of the central layer sandwiched between a pair of electrodes unique to each piezoelectric element. In the '079 patent, the piezoelectric power generator is mounted within a water flow and is allowed to undulate in the presence of turbulent forces in the fluid stream. Repetitive flexures of the piezoelectric elements induce stresses in the piezoelectric materials that the piezoelectric materials convert into electrical energy.
In one embodiment of the '079 patent the flow induces a continuous sine wave shape in the elongated structure including spaced crests and troughs. When this occurs piezoelectric layer on one side of the structure is bent into a convex shape and a dielectric layer on the other side is bent into a concave shape. The concave shaped piezoelectric layer is stressed in tension to create an electrical charge for electrodes in contact with it. Conversely the convex concave shaped piezoelectric layer on the opposite side of the structure is stressed in compression to create an electrical charge for electrodes in contact with it.
Other examples where piezoelectric materials are used to generate energy from a fluid flow include U.S. Pat. No. 4,387,318 issued to Kolm et al. on Jun. 7, 1983, entitled: “Piezoelectric Fluid-Electric Generator” which shows a piezoelectric fluid-electric generator including a piezoelectric bending element, means for mounting the one end of the belt the elements in a fluid system, means for driving the piezoelectric element to oscillate with the energy of a fluid stream and electrode means connected to the piezoelectric ending element to conduct current generated by the oscillatory motion of the piezoelectric bending element. In one embodiment of this type, a paddle like blade extends from one end of the piezoelectric bending element to be deflected by the fluid while the other end is rigidly mounted to a support. Yet another example of a piezoelectric generator is shown in U.S. Pat. No. 7,560,856 issued Jul. 14, 2009. In this patent electrical energy is produced by converting kinetic energy from fluid flow with membranes that generate electrical energy in response to deformation by the fluid flow passing through a piezoelectric medium attached to the deforming membranes. Sets of membranes define variable fluid flow restrictors that oscillate due to interaction of the force of fluid flow and Bernoulli effect.
A variety of performance limitations are associated with such piezoelectric systems. One is that the production of electrical energy requires subjecting the piezoelectric materials to cycles of inducing and relieving stresses in the materials. This can lead to early failure of such piezoelectric materials. Further, there are limits as to the extent to which such piezoelectric materials can generate electricity in response to applied energy.
U.S. Pat. Nos. 7,821,144 and 8,026,619 describe another alternative approach to generating energy from a flow of the fluid that requires neither piezoelectric materials nor spinning blades. Instead, in these cases an energy converter is provided that includes a flexible membrane having at least two fixed ends fixed to a frame. The membrane and frame are arranged in a flow of fluid so that the membrane oscillates when subject to fluid flow. One of an electrical conductor and a magnet is joined to membrane and oscillates with the membrane. The other of the electrical conductor and a magnet is positioned proximate to the path of movement of the membrane and apart therefrom. The oscillation of the membrane caused by the fluid flow causes a relative movement between the electrical conductor and the applied magnetic field inducing a current in the electrical conductor.
Although these systems do not have rotating blades, they require that the membrane oscillates across the path of fluid flow, they create noise and they create vibrations. Accordingly, these systems are also disruptive to the flow of fluid, objects in flow of fluid and to the surrounding environment and have efficiency limits due to the loss of energy to noise and vibration and the requirement that they oscillate in the flow of fluid rather than undulate.
Thus, what are still needed in the art are methods and apparatuses that can generate energy from moving flows of fluid such as wind and water and that can do so with greater efficiency, greater reliability and less disruption of the environment.